1. Field
The disclosure relates to the manufacture of tires for vehicle wheels and in particular to the vulcanizing presses for curing green tire blanks.
2. Description of Related Art
In order to manufacture a tire for a wheel, a green tire blank is first of all created which comprises various elements of raw rubber and reinforcing elements. The green tire blank is then cured in a press to vulcanize the rubber and obtain the tire casing.
It is known practice, for example from document EP-0 638 409 in the name of the Applicant company, to heat the vulcanizing press using electromagnetic induction. In such a context, each inductor may be equipped with a metal armature made of magnetic sheet, sometimes referred to as electrical sheet, in the manner of transformers and the armatures of synchronous motors. Such inductors are used with alternating currents at relatively low frequencies of, for example, between 4 and 1000 hertz, to heat the components of the mold over a large proportion of their thickness. The armature bears a coil and has the shape of a U. In addition, in order effectively to complete the loop of the magnetic field through the component that is to be heated, the inductor is placed in contact therewith. However, this results in two disadvantages.
First of all, the component, which may reach a temperature of between 100 and 400° C., transmits a significant amount of heat to the armature which means that the coil, its insulators and its coatings have to be made from materials able to withstand high temperatures. The alternative is for the inductor to be fitted with cooling means. All of these solutions are very expensive.
In addition, under the effect of the variable magnetic field and of the magnetic hysteresis of the components that are to be heated, which is generally very much greater than that of the magnetic sheets, the inductor is subjected, at a frequency that is twice that of the supply voltage, to a force of attraction and then to a force of repulsion that tends to move it away from the component that is to be heated. This phenomenon may generate movements and impacts of the inductor against the component liable to generate noise in excess of 80 decibels. It also generates wear on those surfaces of the armature that strike the component. Now, the noise constitutes a nuisance to persons in the vicinity and the wear ultimately leads to replacement of the inductors, representing a significant cost and causing production down-time. Admittedly, the inductor can be held firmly against the component using various devices such as springs or pneumatic or hydraulic rams. However, these solutions are expensive without always being effective.
Furthermore, the armatures are built to follow the shape of the component that is to be heated and have planar soles in contact with a planar surface of the component. Nevertheless, the component is heated on just one face, which causes within it an asymmetric expansion that gives it a curved shape. Once that happens, the armature is no longer in stable contact with the component and is pressed against the latter only at the vertex of the curved part. It is therefore liable to move even more by rolling and rocking along its length against the curved part, thereby aggravating the phenomena of noise and of wear. This asymmetric thermal expansion explains why the noise may be low at the start of heating and then reach high values when the temperature of the heated face of the component has increased significantly.